303 research outputs found
Coulomb interaction and ferroelectric instability of BaTiO3
Using first-principles calculations, the phonon frequencies at the
point and the dielectric tensor are determined and analysed for the cubic and
rhombohedral phases of BaTiO. The dipole-dipole interaction is then
separated \`a la Cochran from the remaining short-range forces, in order to
investigate their respective influence on lattice dynamics. This analysis
highlights the delicate balance of forces leading to an unstable phonon in the
cubic phase and demonstrates the extreme sensitivity of this close compensation
to minute effective charge changes. Within our decomposition, the stabilization
of the unstable mode in the rhombohedral phase or under isotropic pressure has
a different origin.Comment: 9 pages, 4 tables, 1 figur
Ab initio phonon dispersion curves and interatomic force constants of barium titanate
The phonon dispersion curves of cubic BaTiO_3 have been computed within a
first-principles approach and the results compared to the experimental data.
The curves obtained are very similar to those reported for KNbO_3 by Yu and
Krakauer [Phys. Rev. Lett. 74, 4067 (1995)]. They reveal that correlated atomic
displacements along chains are at the origin of the ferroelectric
instability. A simplified model illustrates that spontaneous collective
displacements will occur when a dozen of aligned atoms are coupled. The
longitudinal interatomic force constant between nearest neighbour Ti and O
atoms is relatively weak in comparison to that between Ti atoms in adjacent
cells. The small coupling between Ti and O displacements seems however
necessary to reproduce a ferroelectric instability.Comment: 12 pages, 4 figure
Giant direct magnetoelectric effect in strained multiferroic heterostructures
The direct magnetoelectric (ME) effect mediated by lattice strains induced in
a ferroelectric film by a ferromagnetic substrate is evaluated using
first-principles-based calculations. To that end, the strain sensitivity of
ferroelectric polarization and the film permittivity are calculated as a
function of the in-plane biaxial strain for Pb(Zr0.52Ti0.48)O3 films under
various depolarizing fields. It is found that the ME voltage coefficient varies
nonmonotonically with this strain and may reach giant values exceeding 100
Vcm-1 Oe-1 over a strain range that can be controlled through the electrical
boundary conditions.Comment: Accepted as a Rapid Communication by Phys.Rev.B (http://prb.aps.org/
Interfacial Magnetoelectric Coupling in Tri-component Superlattices
Using first-principles density functional theory, we investigate the
interfacial magnetoelectric coupling in a tri-component superlattice composed
of a ferromagnetic metal (FM), ferroelectric (FE), and normal metal (NM). Using
Fe/FE/Pt as a model system, we show that a net and cumulative interfacial
magnetization is induced in the FM metal near the FM/FE interface. A carefully
analysis of the magnetic moments in Fe reveals that the interfacial
magnetization is a consequence of a complex interplay of interfacial charge
transfer, chemical bonding, and spin dependent electrostatic screening. The
last effect is linear in the FE polarization, is switchable upon its reversal,
and yields a substantial interfacial magnetoelectric coupling.Comment: 5 pages, 6 figure
Electron localization : band-by-band decomposition, and application to oxides
Using a plane wave pseudopotential approach to density functional theory we
investigate the electron localization length in various oxides. For this
purpose, we first set up a theory of the band-by-band decomposition of this
quantity, more complex than the decomposition of the spontaneous polarization
(a related concept), because of the interband coupling. We show its
interpretation in terms of Wannier functions and clarify the effect of the
pseudopotential approximation. We treat the case of different oxides: BaO,
-PbO, BaTiO and PbTiO. We also investigate the variation of the
localization tensor during the ferroelectric phase transitions of BaTiO as
well as its relationship with the Born effective charges
Strong influence of the complex bandstructure on the tunneling electroresistance: A combined model and ab-initio study
The tunneling electroresistance (TER) for ferroelectric tunnel junctions
(FTJs) with BaTiO_{3} (BTO) and PbTiO}_{3} (PTO) barriers is calculated by
combining the microscopic electronic structure of the barrier material with a
macroscopic model for the electrostatic potential which is caused by the
ferroelectric polarization. The TER ratio is investigated in dependence on the
intrinsic polarization, the chemical potential, and the screening properties of
the electrodes. A change of sign in the TER ratio is obtained for both barrier
materials in dependence on the chemical potential. The inverse imaginary Fermi
velocity describes the microscopic origin of this effect; it qualitatively
reflects the variation and the sign reversal of the TER. The quantity of the
imaginary Fermi velocity allows to obtain detailed information on the transport
properties of FTJs by analyzing the complex bandstructure of the barrier
material.Comment: quality of figures reduce
Band alignment at metal/ferroelectric interfaces: insights and artifacts from first principles
Based on recent advances in first-principles theory, we develop a general
model of the band offset at metal/ferroelectric interfaces. We show that,
depending on the polarization of the film, a pathological regime might occur
where the metallic carriers populate the energy bands of the insulator, making
it metallic. As the most common approximations of density functional theory are
affected by a systematic underestimation of the fundamental band gap of
insulators, this scenario is likely to be an artifact of the simulation. We
provide a number of rigorous criteria, together with extensive practical
examples, to systematically identify this problematic situation in the
calculated electronic and structural properties of ferroelectric systems. We
discuss our findings in the context of earlier literature studies, where the
issues described in this work have often been overlooked. We also discuss
formal analogies to the physics of polarity compensation at LaAlO3/SrTiO3
interfaces, and suggest promising avenues for future research.Comment: 29 pages, 23 figure
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